Method and System Having Reference Signal Design for New Carrier Types

ABSTRACT

A method and network element for providing reference signals to a user equipment, the method determining a reference signal pattern at the network element; and sending the reference signals to the user equipment using a reference signal mapping based on the reference signal pattern. Further a method and user equipment for receiving reference signals from a network element, the method determining a reference signal mapping at the user equipment; and detecting the reference signals at the user equipment using the reference signal mapping.

FIELD OF THE DISCLOSURE

Reference signals between the network element and a mobile device and inparticular relates to orthogonal frequency division multiplexing (OFDM)reference signals.

BACKGROUND

The 3^(rd) Generation Partnership Project (3GPP) Long Term Evolution(LTE) Standards are being enhanced to achieve better system performancesby utilizing currently available frequency spectrums in a more efficientmanner. The evolution of the LTE is referred to as Long TermEvolution-Advanced (LTE-Advanced). In LTE Advanced, the peak target datarates are 1 Gbps and 500 Mbps for downlink and uplink respectively.

In order to achieve the target data rates, one approach is to usecarrier aggregation (CA) techniques to utilize bandwidth aggregation ofa variety of different arrangements of component carriers (CCs)including the same or different bandwidths, adjacent or non-adjacent CCsin the same frequency band or different frequency band. In order toachieve carrier aggregation enhancements in LTE-Advanced, the 3GPP radioaccess network (RAN) utilize a new carrier type (NCT) scenario foreither stand alone or non-stand-alone carrier type. To deal with thisnew carrier type, one consideration is the reference signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be better understood with reference to thedrawings, in which:

FIG. 1A is a block diagram showing a one antenna port case for a mappingof downlink reference signals utilizing a normal cyclic prefix;

FIG. 1B is a block diagram showing a two antenna port case for a mappingof downlink reference signals utilizing a normal cyclic prefix;

FIG. 1C is a block diagram showing a four antenna port case for amapping of downlink reference signals utilizing a normal cyclic prefix;

FIG. 2A is a block diagram showing time-frequency lattices for specialsubframes for configuration 1, 2, 6 or 7;

FIG. 2B is a block diagram showing time-frequency lattices for specialsubframes for configuration 3, 4, or 8;

FIG. 2C is a block diagram showing time-frequency lattices for all othersubframes than those of FIGS. 2A and 2B;

FIG. 3 is block diagram showing time-frequency lattices for channelstate information reference signals for 2, 4 and 8 port scenarios;

FIG. 4 is block diagram showing an example heterogeneous network;

FIG. 5 is block diagram showing one example of RS mapping for a high andlow density scenario utilizing a fixed mapping method;

FIG. 6 is block diagram showing one example of RS mapping based on CDMfor a high and low density scenario utilizing a fixed mapping method;

FIG. 7 is block diagram showing one example of RS mapping for a high andlow density scenario utilizing a flexible mapping method;

FIG. 8 is block diagram showing one example of RS mapping based on CDMfor a high and low density scenario utilizing a flexible mapping method;

FIG. 9 is a schematic diagram showing an example protocol stack in awireless communication system;

FIG. 10 is a signaling diagram showing the sending of RSs between anetwork element and UE;

FIG. 11 is a block diagram showing a simplified example network element;and

FIG. 12 is a block diagram of an example user equipment.

DETAILED DESCRIPTION OF THE DRAWINGS

The present disclosure provides a method for providing reference signalsto a user equipment from a network element, the method comprising:determining a reference signal pattern at the network element; andsending the reference signals to the user equipment using a referencesignal mapping based on the reference signal pattern.

The present disclosure further provides a network element for providingreference signals to a user equipment, the network element comprising: aprocessor, wherein the processor is configured to: determine a referencesignal pattern at the network element; and send the reference signals tothe user equipment using a reference signal mapping based on thereference signal pattern.

The present disclosure further provides a method for receiving referencesignals at a user equipment from a network element, the methodcomprising: determining a reference signal mapping at the userequipment; and detecting the reference signals at the user equipmentusing the reference signal mapping.

The present disclosure further provides a user equipment for receivingreference signals from a network element, the user equipment comprising:a processor, wherein the processor is configured to: determine areference signal mapping at the user equipment; and detect the referencesignals at the user equipment using the reference signal mapping.

While the present disclosure is described with regards to the 3GPPLTE-Advance New Carrier Type, the embodiments present herein couldequally be applied to other network types and network elements, and thepresent disclosure is not limited to any particular network type orelement.

As used herein, a network element could be any network side entity,including but not limited to an evolved Node B (eNB), access point, basestation, relay, among others.

A user equipment, as used herein, could be any computing devicecommunicating with a network element, and includes but is not limited toa mobile device, a tablet, a laptop, a data enabled cellular telephoneor pager, a personal computer, among others.

Reference Signals

Downlink reference signals are a predefined signal which occupiesspecific resource elements (REs) in the downlink time-frequency lattice.Various types of downlink reference signals exist and are transmittedfor different purposes. For example, in the 3GPP LTE Release 8, thecommon reference signals are designed for time and frequency tracking,channel estimation for channel state information (CSI) feedback and datademodulation, as well as radio resource management (RRM).

A UE, in an initial stage after a cell search is performed, still mayneed to keep track of time and frequency synchronization to the cell tocompensate for the error from a local oscillator or Doppler effectsbased on common reference signals (CRSs).

In the 3GPP LTE Release 10 Standard, the CSI-RS is introduced to supportup to eight transmit antennas and multi-cell cooperative transmissionschemes, such as cooperative multipoint (CoMP) and heterogeneousnetworks (HetNet).

With any reference signals (RSs), in order to correctly estimatewireless channels, a reference signal spacing must satisfy the Nyquistsampling theorem in time and frequency domains. For the time domain, thereference signal spacing is related to the Doppler spread, which isgiven by equation 1 below.

$\begin{matrix}{P_{t} < \frac{1}{2\; f_{d,\max}T_{s}}} & (1)\end{matrix}$

In the above, f_(d,max) and T_(s) are the maximum Doppler frequency andan OFDM symbol duration including guard intervals, respectively. P_(t)is the reference signal spacing on the time domain.

For frequency domain, the spacing is related to the delay spread givenby equation 2 below.

$\begin{matrix}{P_{f} < \frac{N}{L}} & (2)\end{matrix}$

In equation 2, N and L are the number of subcarriers and the maximumnumber of channel delay profiles in an OFDM symbol, respectively. P_(f)is the reference signal spacing on the frequency domain.

As indicated above, various types of reference signals may exist. Theseinclude cell-specific reference signals, UE-specific reference signalsand channel state information reference signals. Each is discussedbelow.

With regard to cell-specific reference signals, in the 3GPP LTE Release8, 9, 10 and 11 standards, cell-specific reference signals are mainlyused for channel quality estimation and channel estimation fordemodulation of control channels and physical downlink shared channel(PDSCH) which does not use a UE-specific reference signal. Further,cell-specific reference signals (CRSs) as well as the primarysynchronization signal/secondary synchronization signal (PSS/SSS) may beused for time and frequency synchronization while the UE is either in aconnected mode or in an idle mode. Existing reference signal receivedpower (RSRP)/reference signal received quality (RSRQ) measurements areperformed over a measurement bandwidth, which is configurable.

Reference is now made to FIGS. 1A, 1B and 1C, which show a mapping ofdownlink reference signals utilizing a normal cyclic prefix.

As seen in FIG. 1A, a time-frequency lattice 110 shows a one antennaport case 110. In FIG. 1B, the time frequency lattices 120 show a twoantenna port case and in FIG. 1C, the time frequency lattices 130 show afour antenna port case.

In each of the cases in FIGS. 1A-1C, a time-frequency lattice isprovided in which reference signals are mapped to specific locations. Asseen in FIG. 1B, with two antenna ports various reference signals 122may be used for a particular antenna port. The same time and frequencyslot in the other antenna port, shown by element 124 is not used fortransmission on that antenna port.

Thus, as shown in FIG. 1C, antenna port numbers 0 to 3 may be used by aneNB to provide four separate channel estimates. For each antenna port, adifference RS mapping pattern has been designed to minimize theintra-cell or inter-cell interference between multiple transmit antennaports.

For example on antenna port 2, designated by reference signal 132, andantenna port 3, designated by reference signal 134, only four referencesignals are provided for these antenna ports, which is half of thenumber for the antenna ports 0 and 1. This is due to the fact that highspeed user equipments (UEs) are unlikely to use all four antenna portsto achieve sufficient channel estimation accuracy.

Reference signal spacing in time and frequency can be determined by themaximum Doppler spread and the delay spread, respectively. For example,assume that the carrier frequency is 2 GHz and the speed of a mobile is500 km/h, then the reference signal spacing on the time domain isT_(c)≈1/(2f_(d,max))≈0.5 in order to satisfy the Nyquist samplingtheorem in the time domain, as described in equation 1 above. Based onthe above, two reference signals per slot are needed in the time domain.

In the frequency direction, one reference signal is inserted into everysix subcarriers on an OFDM symbol. Since the reference signals arestaggered, one reference signal per three subcarriers within eachreference block are provided. The reference signal spacing depends onthe coherent bandwidth, which is related to channel delay spread. Inparticular, in LTE the channel delay spread is based on the 90% and 50%of the coherent bandwidth. Hence, if the root mean squared (rms) channeldelay spread is σ_(τ then) 90% and 50% of coherent bandwidth isB_(c,90%)=1/(50σ_(τ))=20 kHz and B_(c,50%)=1/(5σ_(τ))=200 kHzrespectively, where the maximum channel delay spread is 991 ns. Thus,the spacing between two reference signals in frequency direction is 45kHz.

With regard to UE-specific reference signals, in the 3GPP Release 8,UE-specific reference signals may be transmitted in addition tocell-specific reference signals. The UE-specific reference signals, ingeneral, are used to enable beamforming of the data transmissions tospecific UEs.

Thus, UE-specific RSs are transmitted in the assigned radio resourceblocks (RBs) for UEs on PDSCH transmission. Using the transmittedUE-specific RSs, a UE estimates channels and demodulates the data in thecorresponding RBs. Since the same precoding is applied to the PDSCH datasymbols before transmission, the signalling is not required to inform aUE of the precoding method and precoding parameters.

The pattern of the UE-specific RSs is chosen so that they do not collidewith the cell-specific RSs. Further, the density of the UE-specific RSis half that of the cell-specific RSs to minimize overhead.

In LTE Release 9, UE-specific RSs are defined to also support dual-layertransmission. The dual-layer can be assigned to either one or two UEsdepending on the transmission mode, which is related to single-usermultiple-input multiple-output (SU-MIMO) or multi-user multiple-inputmultiple-output (MU-MIMO). While two spatial layers may be transmittedif one UE is assigned, a single layer from each of two UEs can beassigned if two UEs are selected.

A UE-specific RS can provide the scalability for LTE-Advanced to choosea design for new RSs. This may be used to achieve efficient inter-cellcoordination by choosing a different pattern of cell-specific RSs.Further, since UE-specific RSs in the assigned RBs are for the channelestimation in the time-frequency lattice, the eigen-structure of thetime and frequency channel covariance matrix can give insights into theoptimal pattern of resource elements (REs) for RSs.

In one embodiment, length-2 orthogonal Walsh codes may be adopted tosupport two layers of the UE-specific RSs. Compared to frequencymultiplexing, the code multiplexing scheme may improve the accuracy ofinterference estimations under slow wireless channel environments sincethe same set of RSs may be used regardless of the number of transmittedlayers.

Reference is now made to FIGS. 2A, 2B and 2C, which shows the mapping ofUE-specific reference signals on antenna ports 7, 8, 9 and 10.

FIG. 2A provides time-frequency lattices 210 for the special subframesfor configurations 1, 2, 6 or 7. FIG. 2B provides time-frequencylattices 220 for a special subframe for configurations 3, 4, or 8. FIG.2C provides time-frequency lattices 230 for all other downlinksubframes.

For each case, each of the four antenna ports 240, 242, 244 and 246 caninclude reference signals 250. In the embodiments of FIGS. 2A, 2B and2C, each shows the reference signals in different configurations betweenthe cases. Further, in all of FIGS. 2A, 2B and 2C, on antenna port 7,and on antenna port 8 the reference signals are provided in the samelocation on the lattice, whereas in antenna port 9 and antenna port 10the reference signals are provided in a different location from thefirst two antenna ports but on the same location with respect to eachother.

In LTE Release 10, downlink SU-MIMO transmission is extended to supportup to eight spatial layers by exploiting MU-MIMO transmission schemes.In order to achieve this, the precoded UE-specific RS is further usedfor the corresponding PDSCH data demodulation. Since the same precodingis applied to the UE-specific RS for each layer as the data symbols,explicit control signalling for precoding information regarding theprecoding methods and the precoding parameters is not required to besent to the assigned UE. The UE-specific RSs are designed to avoidoverlapping with the cell-specific RSs and control channels to ensurebackward compatibility and to avoid inter-layer RS interference by usingorthogonal multiplexing.

In Release 10, the UE-specific RS pattern up to 2 layers are identicalto that of the Release 9. The pattern for up to 4 layers is obtained byextending the rank-2 UE-specific RS pattern in a code divisionmultiplexing (CDM)/frequency division multiplexing (FDM) manner. Inother words, the four layers can be divided into two groups of twolayers, and then each group is precoded with length-2 Walsh-HadamardOrthogonal Cover Codes (OCC) in the LTE Release 9. Further, theUE-specific RSs in different groups are frequency multiplexed onadjacent subcarriers. For eight layer transmission, the UE-specific RSstructure is further extended by using a hybrid CDM/FDM methods with twoCDM groups that are precoded by a length-4 Walsh-Hadamard OCC.

In a further embodiment, the reference signal may be a channel stateinformation reference signal (CSI-RS) The cell-specific reference signalis designated for up to four transmit antennas in Release 8 LTE.However, in Release 11, since up to eight transmit antennas aresupported, new reference signals that are called CSI-RSs are provided toenable a UE to estimate and feedback the CSI corresponding to up to 8transmit antenna parts over a whole bandwidth in an eNB.

The CSI-RS transmission is supported in Release 10 LTE for 1, 2, 4 and 8transmit antenna ports as shown with regard to FIG. 3.

Referring to FIG. 3, a first lattice 310 is used for 2 CSI-RS ports andincludes lattice locations for the PDCCH, designated by referencenumeral 312, lattice locations for cell specific reference signals,designated by reference numeral 314 and lattice locations forDemodulation-RS (DM-RS), designated by reference numeral 316.

Further CSI having RS patterns are provided. FIG. 3, the term “Ax”designates the cell index “A” and the antenna port “x”. The codedivision multiplex (CDM) group x is used for antenna ports 0 and 1, theCDM group y is used for antenna ports 2 and 3, the CDM group z is usedfor antenna ports 4 and 5 and the CDM group u is used for antenna ports6 and 7. Thus, in the two antenna port embodiment 310, only group x isused. Further, in embodiment 320 having four antenna ports, both group xand group y are used and for embodiment 330 having eight antenna ports,groups x, y, z and u are all used.

CSI-RS is also designed to enable an UE to estimate the CSI for multiplecells rather than a single serving cell. To design the CSI-RS, thefollowing design criteria may be utilized:

-   -   a. Uniform spacing in the frequency domain    -   b. In the time domain, a minimum number of subframes containing        CSI-RS are used to allow minimal wake-up duty cycle when UE is        in discontinuous reception (DRX) mode    -   c. One RE per RB per antenna    -   d. Orthogonally multiplexed from different antennas within a        cell and from different cells    -   e. Avoid REs used for cell-specific RSs, control channels, and        Rel-10 UE-specific RSs to ensure backward compatibility

The CSI-RS configuration is UE-specific. Thus, CSI-RSs are present onlyin some specific subframes based on a given duty cycle and subframeoffset, which are provided through radio resource control (RRC)signaling.

For rate matching for PDSCH transmissions of a Release-10 LTE, a UEassumes that the PDSCH data is only mapped to surrounding REs while forRelease 9 and 10, the PDSCH transmissions are punctured with the CSI-RStransmission.

Since the CDM approach is used in CSI-RS transmissions, as shown in FIG.3, the channel estimation performance may be improved under acooperative MIMO system. Further, a muting method can be applied toavoid collisions with CSI-RS transmissions from other cells, thusproviding better inter-cell interference coordination.

New Carrier Types

New carrier types (NCTs) have been introduced for carrier aggregation toprovide for better spectral efficiency, improved support forheterogeneous network (HetNet) using low-power remote radio heads(RRHs), and energy efficiency. For example, in an unsynchronized NCT,the remote radio head (RRH) may be deployed in a dense area to enhancethe capacity of the cell, as well as at the cell edge to improve celledge performance. Further, small cells may be deployed over macro cellsby using low-powered RRHs, resulting in a heterogeneous networkscenario, as shown by FIG. 4.

In particular, in FIG. 4 a macro cell 410 includes a macro eNB 412 whichtransmits UEs 420 and 422 within the cell.

A small cell 430 is introduced within macro cell 410 in order to providefor better cell edge performance or to enhance performance in denseareas. The cell 430 may be a pico cell with a range expansion area asshown by reference numeral 432. A pico eNB 434 may thus provide serviceto a UE, for example UE 422, within the coverage area of the pico cell430 or range expansion area 432.

In terms of a heterogeneous network environment, a shared cell IDscenario may be utilized, where legacy carriers on the macro cellsoverlap with additional carriers on pico cells and vice versa. In thiscase, the pico cell may benefit from a reduction in interference due toa minimization of mandatory transmissions. For example, the macro cell410 may be configured as the primary cell and the pico cell 434 may beconfigured as the secondary cell. Dynamic interference coordination maybe performed by dynamically controlling the resource allocation andtransmission power. Further, the overhead may be reduced to the physicaldownlink control channel and CRS by having the UE 422 listen to thecontrol channel of macro cell 412 in order to configure for pico cell434. The new carrier thus has spectral efficiency enhancements.

While the above is described with regard to a pico cell within a macrocell, other options are available. These include relays, femto cells,among other low powered nodes.

In the above, the NCT systems may not require the following channels orsignals:

-   -   a. Physical broadcast channel (PBCH)/Release-8 system        information block (SIB)/Paging    -   b. Primary synchronization signal (PSS)/Secondary        synchronization signal (SSS)    -   c. Physical downlink control channel (PDCCH)/Physical hybrid ARQ        indicator channel (PHICH)/physical control format indicator        channel (PCFICH)    -   d. Cell-specific reference signal (CRS)    -   e. Rel-10 mobility is based on measurements in backwards        compatible Component Carriers (CCs)

Thus, the overhead used with common reference signals may be unnecessaryfor the new carrier types. In particular, the overhead for commonreference signals may be more than 10% of the total available resources.Further, the common reference signals design approach may beconservative under a heterogeneous network scenario since a UE moving at500 km per hour would pass through a small cell very quickly, forexample. Thus, the conservative design for CRS may be unnecessary undera new carrier type and may limit the NCT system spectral efficiency.Further, the NCT designs may be applicable to both non-stand-alonecarriers for carrier aggregation enhancements, for example in Release 11LTE. However, this is not limiting and NCT may be extended tostand-alone cases or cases that do not require backward compatibilitywith Release 8, 9, 10 or 11 of the LTE standards.

In accordance with the present disclosure, a reference signal design isprovided that allows for flexibility and scalability to achieve betterspectral efficiency depending on the characteristics of the cell sites.The reference signal mapping may be either fixed or flexible and varioussignaling may be utilized to indicate to a UE to use the differentreference signal mappings. A reference signal pattern is determined by anetwork element and used for providing reference signals to the userequipment. As used herein, the term “density” is used to indicate thetype of reference signal pattern chosen, and a lower density pattern hasless reference signals than a higher density pattern.

Thus, in accordance with one embodiment of the present disclosure,reference signal overheads are reduced while improving spectral systemefficiency. In other words, the density of RSs for channel statusreports, channel estimation, and time and frequency synchronization canbe configured by depending on wireless channel characteristics in agiven deployment scenario. In one embodiment, a density-reduced RS maybe applied in pico cells or indoor environments due to the lowerdispersive propagation channel and/or to users moving at lower speeds.For example, in a heterogeneous network system, the pico cell uses adensity reduced RS while the macro cell uses an existing RS with normaldensity. However, this is not limiting and other deployment scenariosare possible.

The density reducing RS may be provided for either the non-stand-aloneNCT for Release 11 LTE-Advanced or may be provided for a stand-aloneNCT.

Fixed Reference Signal Mapping

In one embodiment of the present disclosure, a fixed reference signalmapping embodiment is provided. In accordance with the embodiment,channel characteristics are utilized to determine a reference signalmapping. For example, wireless channels of pico-cells or indoorenvironments are less dispersive than those of open or urban areas, andthe cell type may be used as an indicator to use a certain referencesignal mapping. Urban areas served by macro-cells are more dispersiveand corresponding channel coherent time is shorter in a macro-cell thanin these pico or indoor environments. Therefore, the density ofreference signals for such less dispersive wireless channels may bedifferent than for cell sites having a longer delay spread, whileenhancing the system spectral efficiency due to the reduction ofoverheads. For example, for an RS design, criteria can either use acell-specific RS design or a CDM design and can be adapted for amacro-cell scenario.

On the other hand, in heterogeneous network scenarios, the RS mappingsare sub-sampled from that of the macro-cell scenarios depending on thewireless channel characteristics. In this case, sub-sampling ratios maybe signaled using system information at the initial connection of theUE. RSs may be transmitted on specific subframes. In this case, thetransmission period, such as the RSPeriodValue, may be signaled withhigher layer signaling.

The above may be illustrated utilizing an example and reference is nowmade to FIG. 5. The example of FIG. 5 shows a “high density” RS scenario510 and a “low density” RS scenario 520. However, the present disclosureis not meant to be limited to only having two density scenarios and aplurality of density scenarios may be provided in some cases.

Further, the embodiment of FIG. 5 shows a puncturing of one half of thereference signals. However, the use of one half of the signals is meantas an example only and in other cases more than half of the signals maybe punctured and in other cases less than half of the signals may bepunctured.

Referring to FIG. 5, a high density scenario 510 provides for thereference signals, for example in a macro cell. In the example of FIG.5, the proposed RS mapping with two antenna ports is considered withoutcollision with DM-RSs of Release 9 or 10 LTE. In a high density scenario510, for example, the number of RSs is the same as that of Release 8 fora high-density scenario. In particular, RSs for a first antenna port areidentified, for example, with reference numeral 512 and RSs for a secondantenna port are identified with reference numeral 514.

In a low density scenario 520, the number of reference signals is halfof that for high density scenario 510 and, in this case, the first andthird pilot symbol locations are punctured. The example of FIG. 5 ishowever only illustrative and in other cases the second and fourth pilotsymbols may be punctured, the first and second pilot symbols may bepunctured, the third and fourth pilot symbols may be punctured, amongother combinations.

From FIG. 5, to enhance the channel estimation quality, RSs in theprevious subframe, which is placed on the fifth symbol in the secondslot, may be used with a corresponding increase in the computationalcomplexity and memory requirements.

In another alternative, if the density-reduced RS is used on a carrierwith PDCCH, the RS of the 5^(th) OFDM symbol of each slot may beeliminated. The RS of the first OFDM symbol of each slot may be kept tomake sure the UE has RSs for PDCCH demodulation.

Referring to FIG. 6, the figure shows a second example of RS mappingbased on code-division multiplexing. In particular, FIG. 6 shows anexample of RS mapping with four antenna ports based on CSI-RS patterns,which uses CDM approach rather than staggered CRS, to make moreefficient RS design and interface coordination of the multiplecooperative transmission scheme, such as CoMP and HetNet scenarios.

As shown in FIG. 6, a high density scenario 610 may be used, forexample, for a macro cell, whereas a low density scenario 620 may beused, for example, for a pico cell. In the embodiment of FIG. 6, thesignal “Ax” represents the cell index “A” and the antenna port “x”,where “x” is used for antenna port 0 and 1 and “y” is used for antennaport 2 and 3.

FIG. 6 shows half of the reference signals removed in the low densityscenario, freeing up space for other purposes.

Further, as shown in the scenario 620, the RSs don't exist in the firstslot of the subframe. In this case, the RSs in the previous subframe mayoptionally be used to improve accuracy of channel estimates.

The puncturing scheme shown in scenario 620 is however only an example.Other puncturing schemes from the high density scenario 610 may also beapplied.

While the timing division sub-sampling is considered in the examples ofFIGS. 5 and 6, the sub-sampling in the frequency domain is also oneembodiment of the present disclosure. The aforementioned embodimentsmay, in addition, be applied to RS mappings for unsynchronized NCTscenarios in some cases.

Flexible Reference Signal Mapping

A flexible RS mapping scenario may be used depending on the wirelesschannel characteristics and cell site deployment scenarios. Unlike thefixed reference signal mapping as described above, which is sub-sampledfrom a high-density reference signal scenarios for a low-density one,the present embodiment has mapping patterns for low-density scenarioswhich are independent from those of the high-density scenarios.

In other words, the locations of RSs for low-density scenarios aredifferent from those of high-density scenarios. The mapping method may,in some embodiments, be signaled with system information at an initialstage. Alternatively, RSs may be transmitted on specific subframes. Inthis case, the transmission period, such as the RSPeriodValue, may besignaled by higher layer signaling such as the dedicated RRC signalingor the medium access control (MAC) control element.

Reference is now made to FIG. 7, which shows one example for flexiblecell-specific RS mappings having two antenna ports for a low and a highdensity scenario. As with the fixed reference signal mapping scenario,the use of two densities is merely meant as an example and a pluralityof densities could be utilized. Again, as used herein, a high densityscenario merely indicates the use of more reference signals for channelestimations whereas a lower density signal has less reference signals.In the example of FIG. 7, the proposed RSs are mapped without collisionwith DM-RSs of Release 9 or 10 LTE.

Although the RS mapping of scenario 710 is similar to that of theembodiment 510 of FIG. 5, the mapping for the low density scenario 720of FIG. 7 is different from that of embodiment 520 of FIG. 5 to allowfor symmetry of RSs. The symmetry of the RSs and the uniform spacingbetween RSs may reduce the error in the assigned RBs. The mapping methodand density may be configurable depending on the wireless channelstatistics.

In the embodiment of FIG. 7, the RS for the first antenna port is shownwith reference numeral 712, while the RS for the second antenna port isshown with reference numeral 714.

In one embodiment, to enhance the channel estimation quality, RSs in theprevious subframe, which is placed on the 4^(th) symbol in the 2^(nd)slot, may be used).

In the case of flexible reference signal mapping, multiple different RSpatterns may be pre-configured or pre-set to suit different scenarios.For example, one RS pattern may be designed for a macro cell scenarioand one RS pattern may be designed for an indoor low mobility scenario.The different patterns may have different time domain periodicities andfrequency-domain periodicities. The time domain offset and the frequencydomain offset could also be different. Different patterns may have anindex and the index may be signaled to UEs within the cell coveragethrough either broadcast signaling or dedicated signaling such as theRRC signaling or MAC Control Elements. The signaling may come fromeither the macro eNB or from a small cell such as a pico eNB.

When a UE enters a cell or starts to monitor the cell, the UE may obtainthe RS pattern information for the cell and start the measurementprocedures based on the obtained RS pattern information. When a handoveroccurs, the information about the RS pattern may be signaled in thehandover command message, for example.

In one alternative, different patterns may be designed from a common RSpattern through a density reduction on the time domain or frequencydomain. For example, low density pattern may be designed by periodicallyremoving the RSs on the time domain or frequency domain from the highdensity RS pattern. If the high density pattern is transmitted everysubframe, the low density pattern could be transmitted every othersubframe, for example, or every 4^(th) subframe for example, but withthe same pattern in each resource block.

In a further embodiment, the low density pattern could be designedcompletely differently and not derived from a common set. In this case,the pattern may be optimized for different densities and/or scenarios.Extra signaling or standardizations may be required to allow the UE tocorrectly interpret the RS pattern.

Reference is now made to FIG. 8, which shows a CDM based example. As inthe example of FIG. 6, the example of FIG. 8 shows RS mapping with fourantenna ports based on CSI-RS mappings, which use a CDM approach ratherthan staggering CRS. In order to make a more efficient RS design andinterference coordination of the multiple cooperative transmissionscheme, such as a CoMP and HetNet scenario, FIG. 8 shows a high and alow density scenario 810 and 820 respectively.

High density scenario 810 of FIG. 8 is similar to that of the highdensity scenario 610 of FIG. 6.

A low density scenario 820 however reduces the RS density by 50%. Thisratio however may be configurable and 50% is merely meant as an example.

Comparing the embodiments of FIG. 6 and FIG. 8, and in particularscenarios 620 and 820, the location of reference signals is different.In one embodiment the different locations may be made to avoid locationsfrom high density mappings. Further, reference signals may be movedtowards the middle of the slot to improve the channel estimation.

In one embodiment, the RSs in a previous subframe may be used to improvethe accuracy of channel estimates. In alternative embodiments, toprovide better resolution of RS mappings, a five cell index (A-E) may berepeated instead of using a ten cell index (A-J).

Similar to the fixed reference signal mapping of FIGS. 5 and 6,sub-sampling in the frequency domain may also be provided. The above mayalso be applied to RS mappings for unsynchronized NCT scenarios.

Signaling

The RS configuration may be signaled to a UE depending on the scenario.In a density reduced RS scenario for a stand-alone carrier, the RSconfiguration may need to be conveyed to the UE immediately upon powerup of the UE. The UE may need to know the RS configuration at theinitial synchronization in order for the UE to decode the physicalbroadcast channel and other channels. In this case, the RS configurationmay be embedded within the PSS/SSS. To achieve this, the RSconfiguration can be associated with the physical cell identity (PCI)which is carried on the PSS/SSS.

For example, in one embodiment, a separate PCI space may be provided formacro cells and for small cells such as pico cells. In this case, if amacro cell is identified, then the UE may assume a high density scenariowhereas, if a small cell is identified, the UE may assume a low densityscenario. Thus, when the UE obtains a PCI from the PSS/SSS it may knowwhether it is attaching to a macro cell or small cell and assume the RSof either a high density for the macro cell and the RS of a reduceddensity for a small cell.

The distinction between macro cell and small/pico cell is however notmeant to be limiting and in other cases an indicator could be providedto the UE to indicate the type of density that the cell utilizes. Inthis case, some macro cells may be able to use low density scenarioswhereas some pico cells may be able to use high density scenarios, asone example.

If the density reduced RS is applied to a non-stand-alone carrier suchas a non-stand-alone secondary cell, then the RS configuration may bedelivered to the UE through the primary cell RRC signaling, since the UEwill have access to the stand-alone primary cell first.

Reference is now made to Table 1 below.

TABLE 1 CDM-RS-Config information element -- ASN1START CDM-RS-Config-r12::= SEQUENCE {    CDM-RS-r12 CHOICE {       release NULL,       setupSEQUENCE {          antennaPortsCount-r12 ENUMERATED {an1, an2, an4,an8},          resourceConfig-r12 INTEGER (0..31),         subframeConfig-r12 INTEGER (0..154),          p-C-r12 INTEGER(−8..15)          RSMappingRule BOOLEAN % Flexible or Fixed         RSCellInfo INTEGER (0..2) % high, medium, low         RSValuePeriod INTEGER (0..9) % Optional          DensityRatioINTEGER (0..1) % ratio for RSCellInfo       }    } OPTIONAL, -- Need O   zeroTxPowerCDM-RS-r12 CHOICE {       release NULL,       setupSEQUENCE {          zeroTxPowerResourceConfigList-r12 BIT STRING (SIZE(16)),          zeroTxPowerSubframeConfig-r12 INTEGER (0..154)       }   } OPTIONAL -- Need ON } -- ASN1STOP

As seen above, the CDM-RS-Config information element may have variousvalues provided including an RSMappingRule, which indicates whether theRS mapping is flexible or fixed. RSCellInfo may provide an integer from0 to 2 to indicate a high, medium or low density for the RS mapping.However, the use of three values is not limiting and in other scenariosmore or less densities may be utilized.

The RSPeriodValue provides for time domain puncturing for one resource.For example, a value of 0 may indicate every subframe whereas a value of1 may indicate every other subframe and a value of 2 may indicate everyfourth subframe. However, the above are merely meant as examples and theRSPeriodValue could indicate various levels of time domain puncturing.

A DensityRatio may optionally be included in the information elementwhich may indicate the RS cell info. There may be two high densityscenarios with different density ratios with different ratio patterns inthe example of Table 1 above.

The signaling of the RS mapping is typically done between the sameprotocol layer between the network element and the UE. Reference is nowmade to FIG. 9, which shows a simplified architecture for communicationbetween various elements in a system for the control plane. A similarprotocol stack exists for the user plane. In particular, a networkelement such as eNB 910 provides cell coverage to a first area and mayserve a UE 920, which communicates with eNB 910 through wirelesscommunication link 922.

As shown in the example of FIG. 9, each element includes a protocolstack for the communications with other elements. In the case of eNB910, the eNB includes a physical layer 930, a medium access control(MAC) layer 932, a radio link control (RLC) layer 934, a packet dataconvergence protocol (PDCP) layer 936 and a radio resource control (RRC)layer 938.

In the case of UE 920, the UE includes a physical layer 940, a MAC layer942, an RLC layer 944, a PDCP layer 946, an RRC layer 947 and anon-access stratum (NAS) layer 948.

Communications between the entities, such as between eNB 910 and UE 920,generally occur within the same protocol layer between the two entities.Thus, for example, communications from the RRC layer at eNB 910 travelsthrough the PDCP layer, RLC layer, MAC layer and physical layer and getsent over the physical layer to UE 920. When received at UE 920, thecommunications travel through the physical layer, MAC layer, RLC layer,PDCP layer to the RRC level of UE 920. Such communications are generallydone utilizing a communications sub-system and a processor, as describedin more detail below.

Based on the above, reference is now made to FIG. 10, which shows asignaling diagram between a network element 1010 and a UE 1012. Networkelement 1010 may be any network element and can include a macro or picoeNB, for example.

As seen by arrow 1020, the network element determines a density level.The density level may be determined by default, for example in the caseof a macro cell automatically being a high density cell and a pico cellautomatically being a low density cell. In other cases the density levelmay be determined based on a policy, for example, by a networkadministrator such as a carrier. Other examples are possible.

The network element 1010 provides an indication of the density and/or RSmapping to UE 1012 explicitly or implicitly, as shown by arrow 1030. Theindication of arrow 1030 may be an explicit signaling of the RS mapping,for example through a broadcast channel or higher layer signaling. Theindication may also be implicit, for example signaling the network typein the case where a macro cell automatically uses the high densitymapping. Such an implicit indication may include the use of the PCI withthe PSS/SSS, as described above, for example.

The UE 1012 receives and stores the indication and at a future pointreceives RSs that utilize the density mapping, as shown by arrow 1040.The UE then detects the RSs based on the density mapping stored, asshown by arrow 1050.

The above may be implemented by any network element. A simplifiednetwork element is shown with regard to FIG. 11.

In FIG. 11, network element 1110 includes a processor 1120 and acommunications subsystem 1130, where the processor 1120 andcommunications subsystem 1130 cooperate to perform the methods describedabove.

Further, the above may be implemented by any UE. One exemplary device isdescribed below with regard to FIG. 12.

UE 1200 is typically a two-way wireless communication device havingvoice and data communication capabilities. UE 1100 generally has thecapability to communicate with other computer systems on the Internet.Depending on the exact functionality provided, the UE may be referred toas a data messaging device, a two-way pager, a wireless e-mail device, acellular telephone with data messaging capabilities, a wireless Internetappliance, a wireless device, a mobile device, or a data communicationdevice, as examples.

Where UE 1200 is enabled for two-way communication, it may incorporate acommunication subsystem 1211, including both a receiver 1212 and atransmitter 1214, as well as associated components such as one or moreantenna elements 1216 and 1218, local oscillators (LOs) 1213, and aprocessing module such as a digital signal processor (DSP) 1220. As willbe apparent to those skilled in the field of communications, theparticular design of the communication subsystem 1211 will be dependentupon the communication network in which the device is intended tooperate. The radio frequency front end of communication subsystem 1211can be any of the embodiments described above.

Network access requirements will also vary depending upon the type ofnetwork 1219. In some networks network access is associated with asubscriber or user of UE 1200. A UE may require a removable useridentity module (RUIM) or a subscriber identity module (SIM) card inorder to operate on a network. The SIM/RUIM interface 1244 is normallysimilar to a card-slot into which a SIM/RUIM card can be inserted andejected. The SIM/RUIM card can have memory and hold many keyconfigurations 1251, and other information 1253 such as identification,and subscriber related information.

When required network registration or activation procedures have beencompleted, UE 1200 may send and receive communication signals over thenetwork 1219. As illustrated in FIG. 12, network 1219 can consist ofmultiple base stations communicating with the UE.

Signals received by antenna 1216 through communication network 1219 areinput to receiver 1212, which may perform such common receiver functionsas signal amplification, frequency down conversion, filtering, channelselection and the like. A/D conversion of a received signal allows morecomplex communication functions such as demodulation and decoding to beperformed in the DSP 1220. In a similar manner, signals to betransmitted are processed, including modulation and encoding forexample, by DSP 1220 and input to transmitter 1214 for digital to analogconversion, frequency up conversion, filtering, amplification andtransmission over the communication network 1219 via antenna 1218. DSP1220 not only processes communication signals, but also provides forreceiver and transmitter control. For example, the gains applied tocommunication signals in receiver 1212 and transmitter 1214 may beadaptively controlled through automatic gain control algorithmsimplemented in DSP 1220.

UE 1200 generally includes a processor 1238 which controls the overalloperation of the device. Communication functions, including data andvoice communications, are performed through communication subsystem1211. Processor 1238 also interacts with further device subsystems suchas the display 1222, flash memory 1224, random access memory (RAM) 1226,auxiliary input/output (I/O) subsystems 1228, serial port 1230, one ormore keyboards or keypads 1232, speaker 1234, microphone 1236, othercommunication subsystem 1240 such as a short-range communicationssubsystem and any other device subsystems generally designated as 1242.Serial port 1230 could include a USB port or other port known to thosein the art.

Some of the subsystems shown in FIG. 12 perform communication-relatedfunctions, whereas other subsystems may provide “resident” or on-devicefunctions. Notably, some subsystems, such as keyboard 1232 and display1222, for example, may be used for both communication-related functions,such as entering a text message for transmission over a communicationnetwork, and device-resident functions such as a calculator or tasklist.

Operating system software used by the processor 1238 may be stored in apersistent store such as flash memory 1224, which may instead be aread-only memory (ROM) or similar storage element (not shown). Thoseskilled in the art will appreciate that the operating system, specificdevice applications, or parts thereof, may be temporarily loaded into avolatile memory such as RAM 1226. Received communication signals mayalso be stored in RAM 1226.

As shown, flash memory 1224 can be segregated into different areas forboth computer programs 1258 and program data storage 1250, 1252, 1254and 1256. These different storage types indicate that each program canallocate a portion of flash memory 1224 for their own data storagerequirements. Processor 1238, in addition to its operating systemfunctions, may enable execution of software applications on the UE. Apredetermined set of applications that control basic operations,including at least data and voice communication applications forexample, will normally be installed on UE 1200 during manufacturing.Other applications could be installed subsequently or dynamically.

Applications and software may be stored on any computer readable storagemedium. The computer readable storage medium may be a tangible or intransitory/non-transitory medium such as optical (e.g., CD, DVD, etc.),magnetic (e.g., tape) or other memory known in the art.

One software application may be a personal information manager (PIM)application having the ability to organize and manage data itemsrelating to the user of the UE such as, but not limited to, e-mail,calendar events, voice mails, appointments, and task items. Naturally,one or more memory stores would be available on the UE to facilitatestorage of PIM data items. Such PIM application may have the ability tosend and receive data items, via the wireless network 1219. Furtherapplications may also be loaded onto the UE 1200 through the network1219, an auxiliary I/O subsystem 1228, serial port 1230, short-rangecommunications subsystem 1240 or any other suitable subsystem 1242, andinstalled by a user in the RAM 1226 or a non-volatile store (not shown)for execution by the processor 1238. Such flexibility in applicationinstallation increases the functionality of the device and may provideenhanced on-device functions, communication-related functions, or both.For example, secure communication applications may enable electroniccommerce functions and other such financial transactions to be performedusing the UE 1200.

In a data communication mode, a received signal such as a text messageor web page download will be processed by the communication subsystem1211 and input to the processor 1238, which may further process thereceived signal for output to the display 1222, or alternatively to anauxiliary I/O device 1228.

A user of UE 1200 may also compose data items such as email messages forexample, using the keyboard 1232, which may be a complete alphanumerickeyboard or telephone-type keypad, among others, in conjunction with thedisplay 1222 and possibly an auxiliary I/O device 1228. Such composeditems may then be transmitted over a communication network through thecommunication subsystem 1211.

For voice communications, overall operation of UE 1200 is similar,except that received signals would typically be output to a speaker 1234and signals for transmission would be generated by a microphone 1236.Alternative voice or audio I/O subsystems, such as a voice messagerecording subsystem, may also be implemented on UE 1200. Although voiceor audio signal output is generally accomplished primarily through thespeaker 1234, display 1222 may also be used to provide an indication ofthe identity of a calling party, the duration of a voice call, or othervoice call related information for example.

Serial port 1230 in FIG. 12 would normally be implemented in a personaldigital assistant (PDA)-type UE for which synchronization with a user'sdesktop computer (not shown) may be desirable, but is an optional devicecomponent. Such a port 1230 would enable a user to set preferencesthrough an external device or software application and would extend thecapabilities of UE 1200 by providing for information or softwaredownloads to UE 1200 other than through a wireless communicationnetwork. The alternate download path may for example be used to load anencryption key onto the device through a direct and thus reliable andtrusted connection to thereby enable secure device communication. Aswill be appreciated by those skilled in the art, serial port 1230 canfurther be used to connect the UE to a computer to act as a modem or toa power source for charging.

Other communications subsystems 1240, such as a short-rangecommunications subsystem, is a further optional component which mayprovide for communication between UE 1200 and different systems ordevices, which need not necessarily be similar devices. For example, thesubsystem 1240 may include an infrared device and associated circuitsand components or a Bluetooth™ communication module to provide forcommunication with similarly enabled systems and devices. Subsystem 1240may further include non-cellular communications such as WiFi or WiMAX.

The embodiments described herein are examples of structures, systems ormethods having elements corresponding to elements of the techniques ofthis application. This written description may enable those skilled inthe art to make and use embodiments having alternative elements thatlikewise correspond to the elements of the techniques of thisapplication. The intended scope of the techniques of this applicationthus includes other structures, systems or methods that do not differfrom the techniques of this application as described herein, and furtherincludes other structures, systems or methods with insubstantialdifferences from the techniques of this application as described herein.

1-30. (canceled)
 31. A method for receiving reference signals at a userequipment from a network element, the method comprising: determining,based on signaling by the network element, a reference signal mapping atthe user equipment; and detecting the reference signals at the userequipment using the reference signal mapping; wherein the signalingindicates a density, the density being one of a high density and a lowdensity, and wherein the low density indicates fewer reference signalsthan the high density; and wherein the low density mapping is a set ofreference signals that is independent of the high density mapping of theset of reference signals.
 32. The method of claim 31, wherein thesignaling utilizes an information element.
 33. The method of claim 32,wherein the information element is a Code Division MultiplexingReference Signal Configuration information element.
 34. The method ofclaim 31 wherein the signaling provides for a mapping rule to indicatewhether the reference signal mapping is fixed or flexible.
 35. Themethod of claim 31, wherein the signaling provides for a value mapped tothe reference signal pattern.
 36. The method of claim 31, wherein thereference signal mapping is received over broadcast signaling.
 37. Themethod of claim 31, wherein the signaling includes a type of the networkelement.
 38. The method of claim 37, wherein a lower density referencesignal mapping is used for at least one of a small cell; a lower delayspread cell; and a longer coherence time of channels cell.
 39. Themethod of claim 38, wherein the small cell is a pico cell in aheterogeneous network.
 40. The method of claim 31, wherein the referencesignal mapping differs between subframes.
 41. The method of claim 31,wherein the detecting uses reference signals from a previous subframe inaddition to reference signals from a present subframe.
 42. The method ofclaim 31, wherein the mapping is based on a common reference signalmapping.
 43. The method of claim 31, wherein the mapping is based on acode division multiplexing mapping.
 44. A user equipment for receivingreference signals from a network element, the user equipment comprising:a processor, wherein the processor is configured to: determine, based onsignaling by the network element, a reference signal mapping at the userequipment; and detect the reference signals at the user equipment usingthe reference signal mapping; wherein the signaling indicates a density,the density being one of a high density and a low density, and whereinthe low density ratio indicates fewer reference signals than the highdensity; and wherein the low density mapping is a set of referencesignals that is independent of the high density mapping of the set ofreference signals.
 45. The user equipment of claim 44, wherein thesignaling utilizes an information element.
 46. The user equipment ofclaim 45, wherein the information element is a Code DivisionMultiplexing Reference Signal Configuration information element.
 47. Theuser equipment of claim 44, wherein the signaling provides for a mappingrule to indicate whether the reference signal mapping is fixed orflexible.
 48. The user equipment of claim 44, wherein the signalingprovides for a value mapped to the reference signal pattern.
 49. Theuser equipment of claim 44, wherein the reference signal mapping isreceived over broadcast signaling.
 50. The user equipment of claim 44,wherein the signaling includes a type of the network element.
 51. Theuser equipment of claim 50, wherein a lower density reference signalmapping is used for at least one of a small cell; a lower delay spreadcell; and a longer coherence time of channels cell.
 52. The userequipment of claim 51, wherein the small cell is a pico cell in aheterogeneous network.
 53. The user equipment of claim 44, wherein thereference signal mapping differs between subframes.
 54. The userequipment of claim 44, wherein processor is configured to detect byusing reference signals from a previous subframe in addition toreference signals from a present subframe.